Remote metering system



May 5, 1936. E. LGREEN ET AL. 2,039,404

REMOTE METERING SYSTEM INVENTORS ATE'ORNEY May 5, 1936- E. l, GREEN ETAL 2,039,404

REMOTE METERNG SYSTEM Filed March 2l, 1953 4 Sheets-Sheet 2 fr IlAINVENTORS @keel/0,0@ WEZOL BY m ATTORNEY May 5, 1936. E. GREEN ET ALREMOTE METERING SYSTEM 4 Sheets-Sheet 5 Filed March 2l, 1935 lNvEN'roRsEf'iewp* MTM ATTORNEY E. l. GREEN ET AL REMOTE METERING SYSTEM FiledMarch 2l, 1935 4 Sheets-Sheet 4 INVENToRs @gef/wiwi@ B ATTORNEY PatentedMay 5, 1936 REMOTE METERING' SYSTEM Estill I. Green, East Orange, N. J.,and'Warren H. Tidd,

White Plains, N. Y.,

assignors to American Telephone and Telegraph Company, a corporation ofNew York Application March 21, 1933, serial No. 662,002

14 Claims.

This invention relates to arrangements for indicating at a remote pointby electrical translating means the position of one or more movableelements of the magnitude of one or more physi- 5 cal quantities. l

It is an object of the invention to provide a system adapted to theremote metering of such quantities as pressures, levels, positions,flows, voltages, currents, watts, etc.

A method which has heretofore been-used for remote metering is totransmit current of a single frequency whose amplitude has been variedin accordance with the magnitude of a quantity to be remotely metered,and to utilize the amplitude of L the received current to produce anindication of the magnitude of said quantity.. Variation in thecharacteristics of the transmission channel employed will ordinarilyaffect the accuracy of an indication transmitted by this method. It is ao purpose of the present invention to provide a system for remotelyindicating the magnitude of one or more quantities in which the accuracyof the indication will not be affected by variations in the transmittingproperties of the 4channel used.

.In accordance with one embodiment of the inve'4 tion an indication ofthe magnitude of a quantity which varies from time/to time istransmitted as the ratio of the amplitudes of two electricalcurrents ofdifferent frequencies. The

amplitude of the current of varied in accordance with the indication tobe transmitted. The current amplitude of the other frequency istransmitted unvaried. Both currents are transmitted to the locationwhere it is desired to utilize the indication. The magnitude relationbetween the received currents is utilized 1 toy produce an indication ofthe magnitude of said quantity. This indication may alsobe recorded toproduce a continuous register of the magnitude of said quantity.

In another form of the invention it is proposed to transmit threefrequencies, the amplitude of i one of which is modulated in accordancewith the magnitude of a varying quantity. At the receiving location theamplitude relationv betweenthe three currents is utilized to produce anindication of the magnitude of said quantity. i

A further purpose of the invention is to provide means whereby thereadings of a plurality of instruments or the indications of themagnitudes of a plurality of remotely existing quantitles may betotalized and the total continuously indicated on a single dial orrecorded graphically. Still another object of the invention is to proonefrequency is vide means whereby the positions of a pluralit of movableelements or the magnitudes of a plurality of quantities may be indicatedat a remote point over a single communication channel.

While the invention will be deilned in the ap- 5 pended claims, thefeatures of the invention will bemore fully understood from the detailedexposition which follows, read in connection with the accompanyingdiagrams, Figures 1 to 5. Fig.

1 is a diagram showing one embodiment of the 10 remote metering systemusing two frequencies. Fig. 2 is a possible embodiment of a remoteindicating system employing three frequencies. Fig. 3 is a diagram of anembodiment of a system for indicating or recording the total of several15 measured quantities at different remote or local points. Anotherpossible embodiment of such a system is shown in Fig. 4. Fig. 5 shows asystem by means of which indications of the positions of several movableelements or the magnitude of 20 several quantities may be transmittedfrom a remote point over'a single communication channel. Referring toFig. 1, two oscillators Gi and G2 are shown which generate thefrequencies fr and f2. The amplitude of frequency f2 which is trans- 25mitted is varied in accordance with the magnitude of some quantity, suchas pressure, level, position, watt, voltage, current, etc., by thepotentiometer P which is actuated directly or indirectly by a gauge,indicator, or other device I. 30 The modulating element, instead ofbeing a potentiometer, might be a variable magnetic coupling (as shownby T in Fig. 2) or some other suitable device. The variations in themagnitude of the frequency fn need not be directly propor- 35 tional tothe variations in thev quantity to be indicated, but may bear anydesired relation to such variations. A iilter or tuned circuit Fa isinterposed in the system of Fig. 1 between the potentiometer and theline to keep the current of fre- 4o quencyvv f1, which is transmittedunchanged, independent of the variations in f2. The two frequencies frand f2 are applied to a suitable trans- 'r'nissiorrn'edium L which maybe a pair of wires in a. telephone cable, a pair of open wres, a con# 45centric conductor system or a radio circuit which terminates at thedistant receiving point. 'I'he transmission line may includesintermediate ampliers or repeaters as indicated by R in Fig. 1.

At the receiving end the two'frequencies are 50 separated by tunedcircuits or iilters F1 and Fa'. The current of frequency fz is thenpassed through an ampliiler and rectifier AD: and the direct currentresulting is applied to one winding I2 of a differential relay Il.

The rectifier 55 may be of the vacuum tube type or some passive device.Current of frequency fr, after being selected by nlter F1, is passedthrough a variable attenuator VA which may be a potentiometer,

variable magnetic coupling, or other device similar to that at thetransmitting end, or a variable attenuating network such as a T or Ltype network. This current is then amplified and rectified in thedetector AD; and the resultant direct current is applied to the opposingwinding Il of the diiferential relay II. The amplitude of the rectinedcurrent corresponding to the frequency fr is controlled by the variableattenuator which is operated by the motor It. The attenuator is socontrolled by the motor I i as to keep the magnitude of the currents inthe two windings I2 and Il of the differential relay II equal. When asuiiicient'difference exists between the currents in these two windingsthe relay operates, closing one of its contacts. This energizes eitherrelay Il or relay Il, the contacts of said relays being arranged toclose circuits whereby the motor I6 will rotate in one direction or theother. The direction of rotation of motor IO is such that the attenuatorwhich is driven by it through coupling device I l tends to restore theequality of the currents in relay II.

It will now be seen that the position of the variable attenuator Acorresponds at any time to the amplitude ratio of the two receivedcurrents of different frequency. The ratio of the amplitudes of the twocurrents transmitted has been made dependent on themagnitude of theremote quantity. Therefore, the position of the variable attenuator VAalso depends on the magnitude of this quantity. A dial or indicator I'is operated in conjunction with the variable attenuator VA and indicatesits position. 'Ihis dial may, therefore, be calibrated to indicate themagnitude of the quantity measured by I. Thus the indication ofthe'magnitude of the remote quantity will be independent of thevariations in the absolute magnitude of the received currents caused bytransmission changes in the channel, this indication being dependentonly on their relative magnitude.

The attenuator Amight, if desired, be placed in the circuit of fz, thevariations in this case being the reverse of those obtained with thearrangement shown.

Since the system of Fig. 1 depends on the ratio of the currents of twofrequencies, its accuracy may be affected by attenuation changes in thetransmission medium (as, for example, those due to temperaturevariations, leakage changes, re- Deater variations, etc.) which affectthe two frequencies differently. It is quite probable that should suchchanges be present, the desired degree of accuracy could be attained inthe system lwhichhas been described above by choosing the twofrequencies sufficiently close together. Another method of obviatingsuch a diillculty has been devised, however, and a possible embodimentthereof is shown in Fig. 2.

In this system three generators are shown, G1,

-Ch and Ga, generating three different frequencies,

f1, fz and f3. The amplitude of the current of frequency f: is varied inaccordance with the magnitude of the quantity measured by the meter orgauge I. In this case variable magneticcoupling between the two windingsof the transformer T has been used. 'I'he current of frequency j: istransmitted over the line with the currents of the other two frequenciesf1 and ,f3

whose amplitudes have not been altered,

At the receiving location an equalizer EQ is provided so that the lossof the line and equalizer may be made the same at all three frequencieswhen first adjusted. Current of frequency fa is selected by a filter ortuned circuit F3'. and introduced-to an amplifier and modulator AMJ. Afilter or tuned circuit F4 selects a modulation product such as thesecond harmonic of fs, i. e., f4'=2fs. This frequency f4 is thenrectified in the detector AD4 and the resultant direct current isapplied to one winding I2 of the differential relay II.

Al combination of filters or 'tuned circuits F1.: selects the currentsof frequencies f1 and fz and passes them through a variable attenuatorVA to an amplifier and modulator AMM. Another filter or tuned circuit F5selects a modulation product of these two frequencies fs and introducesit to a rectifier ADa. The resultant direct current is impressed on theother Winding I 3 of the differential relay II. 'Ihis relay actuates thecontrol apparatus CA which drives the variable attenuator VA in such away as to maintain the currents in the two windings I2 and I3 of thedifferential relay II equal. The indicator I' shows the position of thevariable attenuator VA and, therefore, as will be presently shown, themagnitude of the indication which was transmitted.

The operation of the system will now be analyzed for a particular caseand there will be described the method of determining the frequencyallocation so that the desired indicationl i1=S1n wit (1) in :sln wat(2) 3=a Sin wat (3) Now assume that the transmission loss caused by theline and equalizer is a constant b at all three frequencies. Also letthere be variations from initial condition in this line loss made up oftwo components, the first of which is constant at all three frequenciesand may be included in the overall line loss b, and the second a loss dwhich depends on the frequency. Now the received currents are expressedas:

i1'=bd1 Sin auf (4) 2'=bd2 Sin wat (5) i3'=abd3 sin wat (6) The variableattenuator VA is now assumed to attenuate the currents of frequencies f1and fz in the ratio c, and the current of frequency f3 is passedunchanged to its modulator Alm. The expressions for the currents at theinput of the amplifier-modulators will be:

Both amplifiers may be assumed to have a gain e which is the same at allthree frequencies. Now let both modulators be of the second order typeand assume that the filter F4 is tuned to select the second harmonic ofla. so that f4=2f.1.

Also assume that the iilter Fs selects a frequency which is the sum ordifference of the two frequencies introduced, so that ls=i1zfa 'I'heamplitudes of the modulation products will then be When these currentsare rectined the direct currents resulting, assuming a square lawmodudator, will have amplitudes:

If now we express the above ratios in decibels* See Decibel-The Name forthe'lransmission Unit" 20 log %+80 log a+80 log b+80 log dri-80 log e=W. H. Martin, Bell System Technical Journal, January 20 los %+80 logb+80 log c+80 log e+40 log d1+40 10g dz (14) If now we let A=20 log aand similarly throughout, we may write Equation (14) This equation willbe satisiied if the following relations obtain:

It will be seen from the above Equation (16). that the indication ondial I which depends on c will always correspond to the transmittedindication, which is represented by a, if the condition expressed inEquation (17) is satisfied. If the law of variation of d with frequencyis known, frequencies may be readily chosen to satisfy this condition,(17). If, for instance, this variation is directly proportional tofrequency, that is D=k/, then since D; is the arithmetic mean of D1 andDr, f3 will also be located midway between fr and ,f2 in the frequencyspectrum. It will also be noticed from Equation (l) that the equivavlent of the line with equalizer or the variation in this equivalentwhich is uniform with frequency will not aiect the accuracy oi thetransmitted indication.

In connection with remote indicating systems of the type alreadydescribed, it may be desirable to obtain or record continuously thetotal of sevproduced on indicators Iz and In. The telemetering systemused may be one'such as is de scribed in this invention or any othersuitable system. Also, if desired, any of the indications I1', In and Iamay be locally produced.

Attached to the indicators I1', Iz' and In are variable resistances VR1,VR: and VB3, which are actuated respectively by.the indicators. Abattery BA1 is shown, the current from which is used to energize thedifferential relay Il. Winding l2 of this relay is energized by thebattery current after it has passed successively through the variableresistances VRa, VR.: and VRx. Current for winding not the relay passesnrst through variable resistance VRA.

Operation or the differential relay 3| energizes either relay Il orrelay 35. I'he contacts of these relays are so connected to the motor 36and the power source that the motor is operated in one direction or theother, depending on the operation of the differential relay. The motor,through some connecting mechanism such as the worm and gear 31, variesthe amount of resistance VRA connected in the circuit. The operation ofrelays 3|, VIl and 3l and motor I8 with driving mechanism 31 is suchthat the amount of resistance VR4 in the circuit is always varied insuch a direction as to restore the equality of the two currents in thedifferential relay 3l. The dial I4 is attached to the variableresistance and always indicates its position.

Since the currents in the two windings of the diii'erential relay aremaintained equal, the lvalue of the resistance VRA connected inthecircuito? winding 33 must be equal to the combined values of theresistances VRi, VR: and VR.: included in -the circuit of winding 32.The dial I4 ma*1 be calibrated similarly to dials I1', In' and Iz' andwill indicate continuously the totals of the readings on these dials.The recording stylus I8 may be driven by the same device and used tomake a graphical record of the readings of the indicator I4 on a roll of-paper driven by a clockwork 39.

Another arrangement which might be used to indicate the total of severalother indications is shown in Fig. 4. Indicators I1'. I2' and I3 whichmay be the indicators of remote quantities or local indicators, operatevariable resistances VR.- VRr and VRa, respectively. These resistancesare connected in series and form one arm of a Wheatstone bridge. Thebalancing arm consists of another varable resistance VRA. The ilxedratio resistances FR.; and FR: complete the bridge circuit. A source ofdirect current is connected to lthe bridge at terminals I and 3, andalso energizes the field of an electric motor ll. The armature of themotor Il is connected to terminals 2 and 4 oi the bridge. The motorcontrols the variable resistance VR4 through connection device 42. Anindicator I4 is operated in conjunction with this resistance VRA.

In operation the ileld of the motor is always energized, but with theresistances so adjusted that the bridge is balanced, there is no voltageimpressed on the armature winding, and the armature will not rotate.However, as soon as the balanced condition is disturbed by a change inthe sum of VH1, VR: and VRa, a voltage difierence will be created acrossterminals 2 and I of VRi-i-VRz-i-VR: than when VR4 is smaller. The motorwill thus always operate in such a direction as to balance the bridgeand when balance has been reached the motor will come to a stop.

channel facilities, to transmit over the same channel a number ofindications such as the readings of several meters located at the sameremote point. This may be accomplished by means of the arrangement shownin Fig. 5. A plurality of indicators I1, Iz, I3 and I4 is shown, each ofwhich is attached to a potentiometer P1, Pz, P3 and P4, respectively. Asin the scheme shown in Fig. 1, two generators are provided, G1 and G1.By means of selectors S1 and Sz, each potientometer is connectedsuccessively into the circuit between G1 and the filter or tuned circuitFa, thus controlling the current amplitude of frequency fz in accordancewith the indications I1, Iz, I: and I4, successively. Currents of bothfrequencies are transmitted through the contacts of relay 46 and overthe line L to the receiving location. Current of frequency fz is hereselected by the filter or tuned circuit Fn', amplified and rectified byADQ, and applied to one winding of the polar relay H. Current offrequency f1 is selected by the filter or tuned circuit F1. By means ofselectors S4 and Ss, this current is transmitted successively throughthe potentiometers P1', Pz', P3' and P4. to thai-amplifier and rectifierAD1, the output voff'wlich is applied to the other winding of the' polarrelay H.

The operation of the system for any single po) sition of the selectors,such as the one shown in Fig. 5, is similar to that of the system shownin Fig. 1. A variation in the magnitude of the current flowing in onewinding of the dierential relay Il will operate this relay and energizeeither relay 52 or relay 53. This applies power from some suitablesource to the motor 58 in such a manner that it will operate in adirection depending on which relay was operated. Through the selector Sein the position shown in Fig. 5 Vthe magnetic clutch CL1 is energized,which couples the motor shaft to the potentiometer P1' and theindicating dial I1. The operation cf the relays causes the motor to varythe position of P1' in such a direction as to restore the equality ofthe two currents in the differential relay Il. Thus, the system may becalibrated so that the indication on the dial I1 is the same as that onthe distant meter I1.

The operation of the selectors which enables the transmission of severalindications over the same communication channel will now be ex plained,referring still to Fig. 5. A small electric motor 49 rotates the disc orcam 48 which has several raised portions or teeth arranged on itsperiphery. Passage of one such raisedportion of the cam opens thecontact 41. Relay 44 is normally energized and through its contacts inthe operated position, relay 45 is also energized. The momentaryinterruption caused by one of the teeth on the disc 48, opening contact41, decnergizesrelay 44 whose armature drops back. This closes thecircuit through the contacts of relay 45, the stepping magnets 4|, 42and 4l, relay 46, and battery. Selectors S1 and Sz are thus advanced tothe second position, connecting potentiOmeter Pz in the circuit;selector S: is advanced and the line circuit is interrupted by relay 46.Relay 45 is of the slow-release type and its armature does not drop backduring the momentary interruption being considered.

`At the receiving location the interruption of the line circuit causesan interruption in the output of the rectitlers AD1 and ADa. Therefore,relay 50, which is in series with the output of AD: and normallyenergized is momentarily deenergized. Relay 5| is also normallyenergized through the contacts of relay 50, and being of theslow-release type its armature does not drop back with the momentaryopening of the contacts of relay 50. In the relaxed position of relay 50the armature closes a circuit through the contacts of relay 5l, thestepping magnets 54, Il, 56 and 5l, and battery. Thus selectors Si andSs are advanced to the second position and the potentiometer P2 isconnected in the circuit. Se-

lector Se is stepped to the second position which energizes the magneticclutch CL2 and couples the motor 58 to the potentiometer Pz and theindicating dial In. Selector S1 is also operated to its second contact.

After the interruption, relay 44 is again energized which allows theline circuit to close and energize relay 50, and the telemeteringoperation continues for the second indication.

This sequence of operations is repeated at intervals determined by thespeed of rotation of the disc 48 until each of the indications has beentransmitted. The last interruption is caused by the raised portion 48-aof the cam 48. This should cause the selectors to return to theirinitial position indicated in Fig. 4. Ii' for any reason, such asfailure of a relay to operate or an accidenta1 interruption of the linecircuit, the selectors at the transmitting and receiving locations arenot in the proper relative positions, the long interruption of contact41 caused by the segment 48-a will restore the selectors to the initialposition automatically. The armature of relay 44 remains relayed duringthe whole inter'- val. This holds the line circuit open which alsoallows relay 50 to relax. Now the armature of relay 45 falls back sincethe interval is longer than its operating delay. Now, if the selectorsat the transmitting location are in any but the initial position, thestepping magnets will be energized through the contacts of selector Sa,the armature and released contact of relay 45, and the contactsconnected with stepping magnet 4I. As soon as the stepping margnets haveoperated the contacts of magnet 43 will be opened, thus deenergizing thestepping magnets. This allows the contacts of 43 to close again, thusreenergizing the stepping magnets if the selector is in any but itsinitial position. This process will be repeated until selector Sa andconsequently the seiectors s1 and s2 are in the initial position shown,

in which position the stepping magnets are not operated again becausethe circuit through selector Sa is not completed. During theseoperations and for a short time afterwards relay 4l will keep the linecircuit open due to its slow-release feature.

In the meantime at the receiving location relay 50 has been deenergizedlong enough to allow the armature of the slow-release relay 5I to dropback in exactly the same manner as at the transmitting end and selectorSi willenergize the stepping magnet if it is in any position but theinitial position shown. The same sequence of events as that at thetransmitting' location will take place until all of the selectors havebeen restored to their initial positions.

When these events have had time to take place relay 4S will release andconnect the transmitting apparatus to the line and the telemeteringprocess will recommence in position I. Meanwhile contact 41 will haveclosed again and the relays 44, 45, 50 and 5| will have taken up theiroperated positions as shown in Fig. 5, ready for the sequence ofoperations to be repeated as has already been explained.

Although this method of transmitting several indications over the samecommunication channel has been described in connection with `thetelemetering system shown in Fig. l', it is equally applicable to thatshown in Fig. 2 or some similar system.

Although the invention has been herein described in connection withparticular embodiments, it will be understood that many modificatIc-ns,both of circuit arrangement and instmmentalities employed, will be madewithout departing from the spirit or scope of the invention as set forthin the appended claims.

What is claimed is: f

l. In a remote indicating system, the method of producing at a distantpoint an indication of the magnitude of a quantity existing at someother point which consists in producing between currents havingdifferent characteristics a current magnitude relation which dependsupon the magnitude of said quantity, transmitting said currents to saiddistant point and utilizing the magnitude relation between the receivedcurrents to produce an indication of the magnitude of said quantity.

2. In a remote indicating system, the method of producing at a distantpoint an indication of the magnitude of a quantity existing at someother point which consists in producing between currents of differentfrequencies a current magnitude relation which depends upon themagnitude of said quantity, transmitting' said currents of differentfrequencies to said distant point and utilizing the magnitude relationbetween the received currents to produce an indication of the magnitudeof said quantity.

3. In a remote indicating system, the method of producing at a distantpoint an indication of the magnitude of a quantity existing at someother point which consists in producing between currents of differentfrequencies a current magnitude relation which depends upon themagnitude of said quantity, transmitting said currents of differentfrequencies to said distant point and utilizing the magnitude relationbetween the received currents to produce an indication of the magnitudeof said quantity, the transmitted frequencies being so selected andutilized that said indication is substantially independent of variationsin the characteristics of the transmitting medium.

4. In a remote indicating system, means for producing between currentshaving different characteristics a current magnitude relation whichdepends upon the magnitude of some quantity, means for transmitting saidcurrents to a remote point, and means at this remote point for utilizingthe magnitude relation between the received currents to produce anindication of the magnitude of said quantity.

5. In a remote indicating system, means for producing between currentsof different frequencies a current magnitude relation which depends uponthe magnitude of some quantity, means for transmitting said currents toa remote point, and means at this remote point for utilizing thekmagnitude relation between the received currents to produce anindication of the magnitude of said quantity.

G. In a remote indicating system,` a plurality of sources of electricalenergy ofy different frequencies, means for varying the amplitude ofenergy from at least one of said sources in accordance with themagnitude of somequantity, means for transmitting vcurrents from saidsources to' a remotev point, and meansat thereceiving location fordetermining and indicating the amplitude relation existing Vvbetweensaid currents, thereby producing anindication of the magnitude of saidquantity.

7. In a remote indicating system, two sources of electrical energy ofdifferent frequencies, means for varying the amplitude of energy fromone of said sources in accordance with the magnitude of some quantity, atransmission medium for conveying currents of both frequencies from thetransmitting location to a receiving location, and means at thereceiving location for determining and indicating the relative amplitudeof the two currents, thereby producing an indication of the magnitude ofsai-d quantity.

8. In a remote metering system, a measuring instrument having a movableelement and a variable attenuator, means to vary the loss ofsaidvariable attenuator in accordance with the position of said movableelement, an oscillation generator, the energy output of which iscontrolled by said variable attenuator, a second oscillation generatorof different frequency whose energy output is constant, means fortransmitting currents of the two frequencies to a remote point, andmeans at said point for indicating the magnitude ratio of the currentsof the two frequencies, thereby reproducing the reading of saidmeasuring instrument.

9. In a. remote metering system, two energy sources of differentfrequencies, means for modulating current from one of said sources inaccordance with the magnitude of some quantity, means for transmittingcurrents from both of said sources to a receiving location, frequencyseparating means, and means for determining and indicating the amplituderelation between the two currents, said determining and indicating meanscomprising, in combination, means for rectifying said currents, meansforcomparing the magnitudes of said rectified currents, and means foradjusting the magnitude of one of said currents prior to rectification,said adjusting means being controlled by said comparing means and beingarranged to produce an indication of the original quantity.

10. In a remote indicating system, three sources of electrical energy,means for varying the amplitude of energy from one of said sources inaccordance with the magnitude of some quantity, a transmission mediumfor transmitting the currents from said three generators, and apparatusat a receiving location for determining and indicating the amplituderelation between the three currents arranged so as to produce anindication 0f the magnitude of said quantity.

i1. In a system for transmitting intelligence in the form of anamplitude relation between currents of different frequency. two sourcesof electrical energy of different frequencies, means for producing anamplitude relation between the currents from said sources which isproportional to some value, means for transmitting said currents througha on medium, and means for utilizing the amplitude relation between thereceived currents to reproduce the value, said frequencies being sochosen that the variations in the eillciency of the transmission mediumat certain of said frequencies substantially compensate simultaneousvariations at others of said frequencies, the said amplitude relation,and therefore the delity of reproduction of said' value, being therebyrendered substantially independent of the variations in thecharacteristics of said medium.

12. In a system for transmitting intelligence in the form of a variationin the amplitude of a frequency, means to supply a plurality of currentsof different frequencies, means to vary the amplitude of one of saidfrequencies, means to transmit, in addition to said frequency, unva riedamplitudes of a second and a third frequency through a transmissionmedium, the said three frequencies being so chosen that theinstantaneous variations in the eiilciency (expressed in decibels) ofthe transmission channel at said ilrst frequency are equal to the meansof the variations in the eillciency of said medium at said second andthird frequencies, the variations at said second and third frequenciesbeing utilized to substantially compensate the variations at said firstfrequency. so that a substantially correct reproduction of thetransmitted variations in the amplitude of said first frequency isobtained.

13. In a remote indicating system, three sources of electrical energy ofdifferent frequencies, means for varying the amount of energy-from oneof said sources in accordance with the magnitude of some quantity, atransmission medium adapted to transmit the currents from said threegenerators, and apparatus, at a receiving location for determining andindicating the amplitude relation between the three currents, therebyproducing an indication of the magnitude of said quantity, saidfrequencies being so related to each other and to the transmissioncharacteristics of said transmission medium-that said indication issubstantially independent of variations in the transmissioncharacteristics of the medium.

14. In a remote indicating system', means for producing between currentsof diierent frequencies a magnitude relation which depends on themagnitude of some quantity, means for transmitting said currents througha transmission medium to some remote point, and means at this remotepoint for utilizing the magnitude relation between the received currentsto produce an indication of the magnitude of said quantity, saidfrequencies being so related to each other and to the characteristics ofthe transmission medium that variations in the characteristics of thetransmission medium at certain of said frequencies, are counteracted byvariations at certain others of said frequencies whereby the fidelity ofthe indication is substantially independent of said variations.

ESTILL I. GREEN. WARREN H. TIDD.

